The use of pH sensors in the diagnosis of particular conditions, in particular ischaemia is known. U.S. Pat. No. 7,813,781 discloses a method and apparatus for determining information concerning ischaemia using a pH sensor. Information may be determined about ischaemia caused by a number of factors, including acute compartment syndrome, vascular disorders, diseases or conditions that affect tissue perfusion including surgical procedures, and generalised diseases or conditions e.g. septicaemia, significant blood loss and significant bodily fluid loss resulting for example from skin bums.
U.S. Pat. No. 6,600,941 and U.S. Pat. No. 6,567,679 both disclose the use of pH measurements of tissue as a system for controlling diagnostic and/or surgical procedures. An apparatus used to perform tissue pH measurements is also disclosed. Real time tissue pH measurements can be used as a method to determine ischaemic segments of the tissue and provide the user with courses of conduct during and after a surgical procedure. When ischaemia is found to be present in a tissue, a user can affect an optimal delivery of preservation fluids to the site of interest and/or use pharmacological methods to improve tissue perfusion and/or effect a change in the conduct of a surgical procedure to normalise the pH within the site of interest.
In general, pH sensors are manufactured several months before use. The pH sensors and associated probes should be stored and used under suitable conditions (−5 to 40 C, 30 to 80% humidity). However, if the conditions for storage or use are unsuitable, the accuracy of the pH sensor may be adversely affected.
From current designs it is known to calibrate pH sensors immediately prior to use. In particular it is known to calibrate pH sensors with at least two standard buffer solutions that span a considerable portion of the range of pH values to be measured (e.g. buffers at pH 4 and pH 10). However, pH sensors may be used in the diagnosis of life threatening conditions such as ischaemia. For such uses, pH sensors must be available for immediate use—where a patient is suffering from a potentially fatal or serious condition, there is insufficient time for complex calibration procedures immediately prior to use. In addition, complex calibration procedures increase the likelihood of contamination and the sterility of the sensor would be compromised.